120 Rev. P. Keith on the Structure of Living Fabrics. 



As there must be an equal cone of rays incident upon the 

 first surface of the crystal, I took other measurements with a 

 view to determine its magnitude. For this purpose I placed a 

 rough micrometer, consisting of two moveable metallic plates, 

 immediately before the lens, and closed the plates, until on 

 looking through the aperture on the second surface I could 

 see them just touching the opposite sides of the circular images. 

 The same thing was done for the interior circle of the annulus, 

 and the focal length of the lens accurately measured. In this 

 manner the extreme dimensions of the conical annulus were 

 ascertained, and the true angle calculated. The mean of 

 three such measurements gave 3° 47' for the corrected angle 

 of the cone. 



It has been observed that the theoretical angle of the cone 

 has been computed from the three indices of refraction as de- 

 termined by M. Rudberg. Now a very small error in the 

 determination of these indices, or a very minute difference be- 

 tween their values in different specimens of the same mineral, 

 would make a considerable change in the angle. On the other 

 hand, the effects of diffraction must in some degree modify 

 the experimental results. And hence, though the measures 

 were not taken with all the means to insure accuracy of which 

 they are susceptible, it will be seen that their correspondence 

 with theory is as close as could be reasonably expected. 



XXII. Of the Structure of Living Fabrics. By the Rev. 

 Patrick Keith, F.L.S. 



[Continued from p. 16.] 



{Fxtcrnal Structure continued.) — The Branches. 

 HPHE Branches are the divisions of the trunk, or caudex 

 -*• ascendcns, originating generally in the upper extremity, 

 but often also along the sides. The primary divisions are 

 again subdivided into secondary divisions, and these again into 

 still smaller divisions till they terminate at last in slender 

 twigs. In their insertion or distribution they are opposite, or 

 alternate, or verticillate, or scattered. In their position they 

 are vertical, that is, lying close to the stein ; or spreading, that 

 is, forming a conspicuous angle with the stem ; or divergent, 

 that is, expanding horizontally; or deflected, that is, hanging 

 down so as to form an arch, as in theWeeping Ash, or Willow. 

 In their size they are proportioned to the dimensions of the 

 trunk, expanding in trees of large growth to a great distance 

 from the centre, and forming a sort of secondary trunk. The 

 horizontal branches of a full-grown Calabash -tree are said to 



